Up to now, there has been a lot of Microsoft Office alternatives. You can have a fully suite product, or you can use a separate application for each purposes. You can also do it offline (usually formed as a product) or you can do it online (this is the new trends that we have right now).
For complete, product, you can choose a lot of alternatives. Some of the popular products are OpenOffice.org, AbiWord, Gnumeric, and KOffice (it's only in GNU/Linux). You can find a lot more if you are willing to find in Google or other search engine.
If you don't want to install any applications, you might want to try other alternatives which is online application and ajax/flash-based. For word processing, you can use AjaxWrite. For spreadsheet, you use Google's product, Google Spreadsheet, and for presentations, you can use Empressr (Thanks to Puji for the information). So, you can have a lot of alternatives out there. It's up to you to decide which application do you want to use.